The article mentioned the Solvay process used to create soda ash from table salt or plant ashes. While it may be possible to extract sodium carbonate from salt, coal ash, and other industrial sources, these techniques are less economical.
There are many naturally occurring sources of sodium carbonate that can be refined through the Solvay process, or elements of the Solvay process, to extract sodium carbonate. Using naturally occurring sources is cheaper than separating sodium carbonate from other manufactured sources (salt), or from toxic and radioactive materials (coal ash).
Soda carbonate containing minerals that miners dredge from alkaline lakes are very easy to extract, and the source is renewable.